A microRNA is a small RNA molecule of 18 to 24 nucleotides and is found in a wide range of eukaryotes. About 1,000 human miRNAs have been identified. The miRNA is a short, single-stranded, endogenously-expressed RNA molecule first reported in 1993. From DNA, an RNA molecule having a loop structure is transcribed, which is called pri-miRNA. The loop is cleaved by an enzyme to yield a pre-miRNA. The pre-miRNA is exported from the nucleus. Then, from the pre-miRNA, a miRNA sequence of about 20 to 25 bases is cut out by Dicer. The miRNA sequence is taken in by an RNA-induced silencing complex (RISC), which is a complex of ribonucleic acid and protein Argonaute. As a result, a miRNA-RISC complex is formed, which binds to the 3′UTR of mRNA and suppresses gene expression. The pairing between a miRNA molecule and a mRNA molecule is not perfect, and therefore a single miRNA molecule can have more than one target gene. This means that a single miRNA molecule can be regulated by targeting a plurality of genes, which is an important feature of miRNA.
It is clearly understood that the microRNA plays important roles in regulation of gene expression in a living organism and that aberration in the microRNA-operated regulation system is related to causes and progressions of many diseases. In particular, microRNAs involved in development and progression of cancer have been investigated in their various aspects, and have received much attention as a promising nucleic acid drug for cancer treatment.
Among the microRNAs involved in development and progression of cancer, the miR-29 family is conventionally known to have an excellent antitumor effect, and have received attention as a candidate molecule for clinical application in microRNA replacement therapy against cancer. For example, Non-Patent Document 1 reports that members of the miR-29 family have functions of regulating tumorigenesis and cancer progression. Non-Patent Document 2 reports that administration of miR-29, the sequence of which is complementary to the sequence of the anti-apoptotic gene Mcl-1, successfully suppressed Mcl-1 expression in a cholangiocarcinoma cell and successfully induced apoptosis of the cholangiocarcinoma cell. Non-Patent Document 3 reports that c-Src and ID1, which are signals related to invasion and growth of cancer, are highly expressed in patients with lung cancer and that miR-29b successfully suppresses invasion and growth of cancer in patients with lung cancer by binding to the 3′-UTR region of ID1 and suppressing the c-Src-ID1 signal. Non-Patent Document 4 reports that administration of miR-29b to acute leukemia patients by using transferrin-binding nanoparticles successfully suppressed expressions of oncogenes such as DNMT1, DNMT3A, DNMT3B, SP1, CDK6, FLT3, and KIT in leukemia cells, indicating an antitumor effect.
Cancer is serious and life-threatening disease, therefore nucleic acid drugs for cancer treatment are demanded for further enhancement of their antitumor effect. Conventionally, microRNAs in living organisms have been studied for their practical use as nucleic acid drugs for cancer treatment. However, no sufficient research has been performed to enhance the antitumor effect of microRNAs in living organisms by mutagenesis of the microRNAs.